Many optical printers use a laser scanning process. The intensity of a laser light beam focused on a two-dimensional photosensitive surface is modulated as the beam is moved relative to such surface to provide a two-dimensional output image at an image zone. In one common system, a rotating multi-faceted polygon is used to line scan a beam of light from the laser across a photosensitive member at the image zone. An acoustooptic modulator intensity modulates the beam in accordance with the gray or brightness level of pixels of a digital image held in a frame store memory. The rotating polygon has the advantages of high generation rates, high resolution and relatively maintenance-free operation.
The mirror facets when assembled will produce artifacts in a recorded image which are known as "banding". It is known that these artifacts are caused by tilt or pyramidal errors in the facets. High quality laser printers commonly incorporate an optical correction system for these pyramidal errors. A cylindrical correcting lens is used in the printer described in U.S. Pat. No. 4,040,096 to Starkweather, issued August, 1977. The lens which is disposed between a polygon and a photosensitive member has power only in one direction (page scan) and no power in a perpendicular direction (line scan). U.S. Pat. No. 4,247,160 to Brueggemann issued January, 1981, discloses a laser polygon printer which has a positive cylindrical mirror disposed between a polygon and a photosensitive member to correct for pyramidal errors. Both these laser printers print only one output image size. The current state of the art in laser color printing is to employ a constantly rotating polygon scanner which scans a fixed line length and uses a fixed number of pixels. The page length is also fixed. In another printer, the line scan length is adjusted by varying timing of the information modulation of the laser beam without adjusting any of the printer's optical elements or the angular velocity of the polygon. The page scan dimension is changed by adjusting the velocity of the photosensitive member. In another example, to reduce the line scan length by one-half, without changing sampling time, the numerical value of every other digital image pixel of a line in frame store memory has been used to information modulate the beam. Since only one-half of the digital pixels are used, this process results in a significant loss of information in the output image. This is particularly unsatisfactory if colored prints are to be made. If optical means are used to change the line scan length, then drastic changes have to be made to the optical system. Also, provision should be made for pyramidal error correction at the different output image sizes.